The present invention relates to hydraulic machines, such as water turbines, pumps and reversible pump-turbines, and particularly to the method of operating a hydraulic machine when it is desired to have the runner rotated within compressed air, which compressed air will depress the water level within the draft tube so that the runner chamber will be filled with compressed air, when the guide vanes are closed.
In operations such as condenser operation, turning reserve operation and starting operation as a pump, the runner of the hydraulic machine is rotated within compressed air filling the runner chamber. In order to prevent heat elevation due to idling of the runner, it has been thought that a large quantity of cooling water must be fed between the rotating portion and the stationary portion of the hydraulic machine during such operation. In order to feed this large quantity of cooling water between such portions by leaking a large quantity of water from the side gaps of the guide vanes, the water pressure within the casing of the hydraulic turbine has heretofore been maintained at a value much larger than the pressure of the air introduced into the runner chamber, and a feed pipe has been provided to supply the cooling water. At the same time, in order to prevent air admission into the casing during the depression of the water level with compressed air within the runner chamber, the hydraulic machine has been provided with a water feed pump or a bypass with a valve between the upper and lower reaches of the penstock, with respect to the inlet valve within the penstock.
Such large quantities of water supplied to the peripheral portion of the runner brings about a cooling effect, however at the same time, it also generates a large quantity of heat due to the torque loss resulting from such a large quantity of water engaging the runner; therefore, adjustment of the quantity of water supplied has become more difficult as the peripheral velocity of the runner becomes larger, based upon a higher head and larger capacity of the machine. Furthermore, due to such a large torque loss related to the large quantity of cooling water, it has been necessary to provide such a hydraulic machine with a starting motor that has a very large output.